A melting furnace for glasses or ceramics is typically constructed as a refractory-lined chamber having an orifice at the bottom from which molten material may be drawn from the furnace. The orifice is subjected to extreme temperatures and has abrasive material flowing through it and must therefore be made of a material which is resistant to abrasion, corrosion and wear. In addition, it is desirable to make the orifice out of a thermally and electrically conductive material, since electric power is typically supplied to the orifice to control the temperature of material flowing through it.
Orifices made of pure tungsten offer good wear resistance but are heavy and difficult to machine and therefore quite expensive to produce. Those made of pure molybdenum are lighter and easier to machine but are less wear-resistant and must be frequently replaced. Because melting furnaces operate at very high temperatures, it is most economical to operate them continuously, thereby avoiding unnecessary energy consumption associated with interruptions and cooldowns. It is very costly to shut down a furnace and empty it in order to replace a worn or disintegrated orifice. Thus, it would be advantageous to have orifices which are durable and rugged.
Composite orifices, as defined herein, are those which are made of at least two dissimilar materials, such as, for example, tungsten and molybdenum or tungsten and iridium. Composite orifices may be made using powder metallurgical techniques in combination with other fabrication processes. Typically, the core and the shell portions are fabricated separately and then press-fit together. For example, the shell portion may be made by powder metallurgical techniques and sintered around a solid metal core to form a tight fit as the shell portion shrinks during sintering. However, unless the core and shell parts are actually bonded together in some way, the core is likely to loosen during furnace operation.
It would be an advancement in the art to provide an efficient and economical method of making a durable wear- and abrasion-resistant composite orifice for a melting furnace.